1. FIELD OF THE INVENTION
The present invention relates to an ignition system for internal combustion engines in which the primary current of an ignition coil is interrupted by an ignition circuit means on the basis of an ignition signal supplied from an electronic control unit.
2. DESCRIPTION OF THE RELATED ART
Prior art ignition systems of this type are constructed so that an ignition circuit means generates a monitor signal for monitoring whether the ignition circuit means is operating normally or not on the basis of an ignition signal from an electronic control unit. In particular, there has been proposed a structure in which, in order to monitor a short-circuiting fault of an ignition coil, an ignition monitor signal is set when the primary current of the ignition coil reaches a first reference level after the conduction of the primary current has been started, and the ignition monitor signal is reset when the primary current reaches a second reference level higher than the first reference level. An example thereof is provided by an ignition system for internal combustion engines disclosed by JP-A-6l-255275.
However, since, in the above-mentioned conventional ignition system, an ignition monitor signal is set when the primary coil current reaches a first reference level after the primary coil current has started flowing, there is a problem that, if the primary coil current is interrupted at an ignition timing immediately after the primary coil current has reached the first reference level as is the case with an abrupt change in the engine speed (at the time of acceleration or deceleration), the ignition monitor signal is reset immediately, and hence it is not possible to assure a sufficient pulse duration for the ignition monitor signal.
Further, in such a case that a power transistor is turned on for the purpose of circuit protection when a spark plug fails to spark, for example, there is a problem that an ignition monitor signal is generated erroneously in the absence of an ignition signal.